1. Field of the Invention
The invention relates to internal visual inspection of turbine engines, such as combustion/gas turbine engines or steam turbine engines by insertion of optical cameras through internal passages within the engine via a delivery mechanism, such as an elongated articulated arm or flexible conduit. More particularly the invention relates to borescopes having foveated image optical heads, with a central first image portion having higher magnification and smaller field of view that is included within and subtended by a second image portion having a wider field of view and lower magnification. The foveated image optical head facilitates maneuvering the borescope optical head through the turbine engine with the wider field of view second image portion to areas of interest, then performing a more detailed inspection by viewing the higher magnification first image portion.
2. Description of the Prior Art
Steam or combustion turbine engines in electric power generation plants require periodic scheduled inspection and maintenance. Gas or combustion turbine engines are more frequently subject to intermittent operation to meet fluctuating power demands; hence for both economic and operational efficiency it is desirable to perform periodic visual internal inspections of the assembled engines between power demand cycles.
A common way to perform visual inspection of blades and vanes in compressor and turbine sections of the engines is by insertion of a rigid, articulate arm or flexible conduit borescope through inspection ports that are constructed about the engine periphery. Many of those inspection ports have inner diameters of less than 13 mm. Inside the engine internal passageways from the inspection ports into many areas of interest have tight confines that physically limit maneuvering space for borescope optical heads. As a practical matter optical heads having axial length greater than 40 mm cannot be maneuvered through confines of some turbine internal passageways.
Optical head dimensional envelope constraints also constrain optical performance capabilities. Image field of view (FOV) and magnification are interrelated. Generally, for a given lens or lens train design increasing FOV decreases magnification. It is desirable to maneuver a borescope optical head within the inspected engine with a relatively wide field of view lens train, but the resultant magnification may not be sufficient to perform a satisfactory visual inspection once the optical head is positioned to an area of interest, for example inspecting for cracks or spallation of a turbine blade airfoil external surface thermal barrier coat. One previously known inspection magnification solution was to remove the lower magnification borescope and replace it with a higher magnification, smaller FOV borescope where needed for more detailed inspection. Another previously known solution for turbine engine internal inspection has been use of a variable FOV/magnification borescopes, but many have optical heads that are too large to fit within the small dimensional confines of desired inspection ports. For example, borescopes have been constructed with separate, dedicated completely separate parallel optical paths or beam splitting downstream of the primary objective lens for different FOV/magnification requirements: essentially two separate optical instruments in a shared optical head. Generally the parallel optical path optical heads have either larger diameter or axial length than allowable for insertion into many turbine engine inspection ports. Other borescopes have zoom magnification capability by physically varying axial spacing between lenses in the lens train and/or the image detector at the design cost of optical head added axial length. Yet other borescopes rely electronic image processing to substitute for physical lens FOV/magnification adjustment. The electronic image processing can include: (i) separate dedicated sections of a common electronic detector for each FOV/magnification setting or (ii) post image gathering enlarged pixel display. Either of those electronic imaging processing techniques decreases image detail. In the case of (i) less than the entire available image sensor pixel density is used to provide image detail. In the case of (ii) the enlarge pixels do not contain any additional image detail compared to a true magnified image view. There has also been concern that inclusion of electronic devices, such as lens autofocus mechanisms or adjustable lens shaping mechanisms, in optical heads that are exposed to heated ambient temperatures within cooling engines might lead to failure of those electronic mechanisms.